This invention relates to a demodulation system for use in demodulating a modulated signal into a pair of output digital signals. It should be noted throughout the instant specification that the modulated signal is subjected to multi-level quadrature amplitude modulation and can specify multiple levels. Accordingly, such a modulated signal will often be referred to as a multi-level quadrature amplitude modulated signal or a quadrature amplitude modulated signal.
In a microwave digital communication system, it is a recent trend that the levels of the quadrature amplitude modulated signal increase in number in order to carry out communication at a high efficiency. Such an increase of the number of the levels makes the quadrature amplitude modulated signal weak against various noises inflicted during transmission. In the demodulation system, any countermeasure must be taken against fading and distortions which may probably occur either singly or collectively in a transmission path.
A conventional demodulation system comprises a transversal equalizer responsive to a system input signal subjected to the quadrature amplitude modulation for equalizing the system input signal into an equalized signal. Supplied with the equalized signal as the modulated signal, a demodulator demodulates the modulated signal into first and second sets of demodulated digital signals by carrying out coherent detection.
With this structure, the transversal filter serves as the countermeasure against the fading and distortions while the coherent detection serves to reduce an influence of noises.
On carrying out the coherent detection, a reference carrier wave must be faithfully reproduced in the demodulator. In addition, jitter components should be radically reduced in the reference carrier wave.
In Japanese Unexamined Patent Publication No. Syo 57-131,151, namely, 131,151/1982, a proposal is offered for a carrier wave reproduction circuit which can reduce jitter components of the reference carrier wave by the use of a phase error signal which is produced by logically processing intermediate digital signals supplied from analog-to-digital (A/D) converters of the demodulator.
In the meantime, an amplitude of the quadrature amplitude modulated signal must be always kept at an optimum amplitude so as to reproduce the reference carrier wave of a high quality. To this end, automatic gain control (AGC) is usually carried out in the demodulator in cooperation with reproduction of the reference carrier wave.
According to the inventors' experimental studies, the carrier wave reproduction circuit is disadvantageous in that establishment of synchronization becomes difficult when the automatic gain control is insufficient and when an in-phase interference or distortion occurs in the transmission path. As a result, such difficulty requires a long time so as to establish the synchronization and often makes establishment of the synchronization impossible.
On the other hand, the transversal equalizer has a critical value of ability of equalization which will be called a first critical value and which is defined when the distortions, namely, intersymbol interferences vary from a small state thereof to a large one. The transversal equalizer has another critical value of ability of equalization that will be called a second critical value and that is defined when the transversal equalizer is recovered from an inoperable state to an operable state by decreasing an amount of distortions. Ideally, the second critical value is equal to the first critical value. Practically, the second critical value is, however, extremely smaller than the first critical value. This means that a long time is required in recovering the transversal equalizer into the operable state once the transversal equalizer is put into the inoperable state.